I'm currently having an EOS 550D with a kit lens. I'm about to update my lens to a new all-round lens one and I'm now considering either to get an

Canon EF-S 15-85mm f/3.5-5.6 IS USM vs L-lens or
1 to 2 L-lenses

What should I do?! After testing the Canon EF-S 15-85mm f/3.5-5.6 IS USM with my EOS 550D I did not see that much improvement over the kit lens in means of image quality. That means, that I will not be spending the 750 euros on that lens if I can get considerable better image quality using an L-lens (for double the price).

So, I want to get excellent image quality for a reasonable price. So my question is; are the euros spent on an L worth it in means of image quality? And what L would you recommend as an all-round lens?